Construction machines such as hydraulic shovels drive a hydraulic pump using the engine, as its power source, that uses fuel (gasoline, light oil, etc.) and generate hydraulic pressure, thereby driving hydraulic actuators (hydraulic motor, hydraulic cylinder, etc.). Being small-sized, lightweight and capable of outputting high power, the hydraulic actuators are widely used as actuators for the construction machines.
Meanwhile, recently proposed are construction machines that employ an electric motor and an electricity storage device (battery, electric double layer capacitor, etc.) and thereby realize higher energy efficiency and more energy saving compared to the conventional construction machines employing hydraulic actuators only (Patent Literature 1).
Electric motors (electric actuators) have some excellent features in terms of energy, such as higher energy efficiency compared to hydraulic actuators and the ability to regenerate electric energy from kinetic energy at the time of braking. The kinetic energy is released and lost as heat in the case of hydraulic actuators.
For example, the Patent Literature 1 discloses an embodiment for a hydraulic shovel having an electric motor as the actuator for driving the swing structure.
Further, to make the best use of superior characteristics of the conventional hydraulic motors (reliability, feeling, etc.) while also realizing the energy saving, there have also been proposed construction machines that are equipped with both a hydraulic motor and an electric motor and drive the swing structure with total torque of the hydraulic motor and the electric motor (Patent Literature 2). The Patent Literature 2 discloses an energy regeneration device for a hydraulic construction machine in which the electric motor is connected directly to the swinging hydraulic motor (hydraulic motor for driving the swing structure). A controller in the energy regeneration device determines the output torque of the electric motor in response to the operation amount of the control lever and sends an output torque command to the electric motor.
Incidentally, such construction machines having a swing structure (e.g., hydraulic shovels) are equipped with a brake for holding the swing structure at a fixed position when the vehicle (construction machine) is stopped. The brake is released when the swinging hydraulic motor drives the swing structure according to a signal inputted from the swing control lever. The brake is activated and works when the lever is returned to the neutral position and the hydraulic motor is stopped. Since the revolution speed (number of revolutions) of the hydraulic motor is not detected in ordinary hydraulic shovels, a hydraulic timer circuit (using a throttle, etc.) is employed in many cases so that the brake operates a while after the lever is returned to the neutral position, as disclosed in Patent Literature 3, for example.